Molecular dynamics study of the primary ferrofluid aggregate formation

Investigations of the phase transitions and self-organization in the magnetic aggregates are of the fundamental and applied interest. The long-range ordering structures described in the Tománek's systematization (M. Yoon, and D. Tománek, 2010 [1]) are not yet obtained in the direct molecular dynamics simulations. The resulted structures usually are the linear chains or circles, or, else, amorphous (liquid) formations. In the present work, it was shown, that the thermodynamically equilibrium primary ferrofluid aggregate has either the long-range ordered or liquid phase. Due to the unknown steric layer force and other model idealizations, the clear experimental verification of the real equilibrium phase is still required. The predicted long-range ordered (crystallized) phase produces the faceting shape of the primary ferrofluid aggregate, which can be recognized experimentally. The medical (antiviral) application of the crystallized aggregates has been suggested. Dynamic formation of all observed ferrofluid nanostructures conforms to the Tománek's systematization.     

Ferrohydrodynamical fundamentals of ferrofluid Bitter pattern evolution

The theory of ferrohydrodynamics describes the specific physical properties associated with a magnetizable fluid. From this theory a constitutive equation was derived, which characterizes the stray-field-induced formation of ferrofluid Bitter patterns on the surface of a ferromagnetic specimen.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Electric field induced pattern formation in interfacial metal colloid films

First results on electric field induced aggregation in a novel colloidal film are presented. The aggregate appears as a dark structure on the shiny background of the metallic colloidal film that forms at the interface between two immiscible liquids. A variety of shapes are observed, ranging from compact circular, to highly ramified dendritic ones, depending on control parameters such as voltage and chemical composition. The aggregation process was investigated for a wide range of parameters by video photography and subsequent pattern analysis, as well as by Auger spectroscopic measurements of dried samples. A new type of mechanism—a combination of electroaggregation of the colloid particles and electrodeposition of metal ions—is proposed to account for the observations.     

Dynamical pattern formation for diblock copolymer films under stretch by moving walls

We propose a computational method that takes into account the dynamical influence of moving rigid walls over the pattern formation for thin films of diblock copolymers. The competition between the surface field energy and elastic stretching energy, and the effects of the molecular relaxation on pattern formation are studied. Finally, it is also observed that stretching the film enhances the ordering of patterns in it.     

Dealloying of platinum-aluminum thin films: dynamics of pattern formation

The application of focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films allows for an in-depth analysis of the dominating physical mechanisms of nanoporosity formation during the dealloying process. The porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS and FIB analysis yields that the porosity evolution has to be regarded as superposition of two independent processes, a linearly propagating diffusion front with a uniform speed and a slower dissolution process in regions which have already been passed by the diffusion front. The experimentally observed front evolution is captured by the Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP). The slower dissolution is represented by a zero-order rate law which causes a gradual porosity in the thin film.     

Comparative study of optical analysis methods for thin films

Three popular optical analysis methods (the transfer-matrix method, the Tinkham formula, and Beer's law) have been used for analyzing the optical spectra of thin films. While the transfer-matrix method is an accurate method, the Tinkham formula and Beer's law are approximate methods. Here we investigated the three methods using measured transmittance spectra of insulating transition-metal dichalcogenide (TMD) thin films on a quartz substrate. Three different semiconducting 2H-TMD systems (MoS₂, MoSe₂, and MoTe₂) were measured and analyzed. The optical conductivities obtained from the measured transmittance spectra using the transfer-matrix method and Tinkham formula and the absorption coefficients obtained using the transfer-matrix method and Beer's law were compared. The comparisons show some discrepancies. The reasons for the discrepancies between the results obtained via the two different methods were examined and the application limitations of the Tinkham formula and Beer's law were discussed.     

The use of colloidal ferrofluid as building blocks for nanostructured layer-by-layer films fabrication

The electrostatic layer-by-layer technique has been exploited as an useful strategy for fabrication of nanostructured thin films, in which specific properties can be controlled at the molecular level. Ferrofluids consist of a colloidal suspension of magnetic grains (with only a few nanometers of diameter) with present interesting physical properties and applications, ranging from telecommunication to drug delivery systems. In this article, we developed a new strategy to manipulate ferrofluids upon their immobilization in nanostructured layered films in conjunction with conventional polyelectrolytes using the layer-by-layer technique. We investigated the morphological, optical, and magnetic properties of the immobilized ferrofluid as a function of number of bilayers presented in the films. Ferrofluid/polyelectrolyte multilayers homogeneously covered the substrates surface, and the magnetic and optical properties of films exhibited a linear dependence on the number of bilayers adsorbed.     

Regularities in the formation of barium oxide films on tungsten tips by field methods

The effects of increasing quantities of barium oxide deposited on the surface of a tungsten tip are monitored by analyzing field-emission images, I-V characteristics and electron energy spectra. Changes are observed in the emission activity of the system throughout the entire adsorption time, as well as the appearance of nonlinear I-V characteristics and a shift in the position of the spectra relative to the Fermi level of the tungsten substrate after the amount of adsorbed barium oxide on the W surface reaches a level that depends on the substrate face. The experimental data set forms the basis of a model for the creation of multilayer films of BaO on W. Fiz. Tverd. Tela (St. Petersburg) 39, 1476–1478 (August 1996)     

Method for the formation of graphene films

A method is proposed to form graphene films using thermodiffusion of carbon atoms from an amorphous carbon or silicon-carbon film with a nanosized thickness through a catalyst film, their accumulation at the catalyst layer/barrier layer interface, and the subsequent carbon quasi-liquid-graphene phase transition. One of the advantages of this method of producing graphene films is the possibility of their formation directly on a dielectric layer and the subsequent suspension of a graphene film over the substrate surface using membrane technologies, which excludes the necessity of using complex procedures to separate a graphene film from the substrate.     

Poling pattern for efficient frequency doubling of Gaussian beams

The commonly used periodic patterning for quasi phase-matching plane-waves is theoretically adequate for completely depleting and transferring a fundamental wave to its second harmonic. However, when working with Gaussian beams the conversion efficiency of such a design lacks due to the inherent Gouy phase shift and the spatially varying beam profile, yielding roughly 88?% conversion efficiency. In this paper, we study the possibility of adding a linear chirp and Gouy-phase shift compensation to the periodic poling. We demonstrate that this poling pattern enables us to achieve near-optimal frequency doubling efficiency of up to 97?%.     

Low-field DC-magnetization study of Ho-doped Mn–Zn ferrite ferrofluid

The physical and magnetic properties of magnetic nanoparticles are crucial for their effectiveness and reliability in biomedical applications. In this article, we report the synthesis of a stable Ho-substituted Mn–Zn ferrite ferrofluid and its physical and magnetic properties. Substitution by rare earth metal plays an important role in determining the magneto-crystalline anisotropy in 4f-3d inter-metallic compounds. Ho³⁺ substitution not only enhanced the magnetic anisotropy but also produced strong spin frustration at low temperature. The field dependence of blocking temperature shows H^2/3 dependency in the entire range of field, i.e., 10–700 Oe, indicating the emergence of Ising spins characteristics in the present system.     

Laser-based techniques for living cell pattern formation

In the production of biosensors or artificial tissues a basic step is the immobilization of living cells along the required pattern. In this paper the ability of some promising laser-based methods to influence the interaction between cells and various surfaces is presented. In the first set of experiments laser-induced patterned photochemical modification of polymer foils was used to achieve guided adherence and growth of cells to the modified areas: (a) Polytetrafluoroethylene was irradiated with ArF excimer laser (λ=193 nm, FWHM=20 ns, F=9 mJ/cm²) in presence of triethylene–tetramine liquid photoreagent; (b) a thin carbon layer was produced by KrF excimer laser (λ=248 nm, FWHM=30 ns, F=35 mJ/cm²) irradiation on polyimide surface to influence the cell adherence. It was found that the incorporation of amine groups in the PTFE polymer chain instead of the fluorine atoms can both promote and prevent the adherence of living cells (depending on the applied cell types) on the treated surfaces, while the laser generated carbon layer on polyimide surface did not effectively improve adherence. Our attempts to influence the cell adherence by morphological modifications created by ArF laser irradiation onto polyethylene–terephtalate surface showed a surface–roughness dependence. This method was effective only when the Ra roughness parameter of the developed structure did not exceed the 0.1 micrometer value. Pulsed laser deposition with femtosecond KrF excimer lasers (F=2.2 J/cm²) was effectively used to deposit structured thin films from biomaterials (endothelial cell growth supplement and collagen embedded in starch matrix) to promote the adherence and growth of cells. These results present evidence that some surface can be successfully altered to induce guided cell growth.     

A mathematical model for pattern formation in biological systems

A system of reaction-diffusion equations with applications in biological problems is presented and studied theoretically and numerically. Solutions demonstrating the following types of behaviour have been obtained: (i) time constant, spatially inhomogeneous, (ii) time periodic, spatially homogeneous, and (iii) time periodic, travelling wave-like. Some applications in embryological problems are discussed.     

Comparative study of ZnO thin films deposited by various methods for use as sensors

Good and adhesive semiconducting films of ZnO (∼ 100–1100 nm) were deposited over planar borosilicate glass by spray pyrolysis and dip & dry method. The films were characterized by X-ray diffraction and optical absorption measurements. The band gap of these films were found to be 3.21 eV and the films were randomly oriented having average crystallite sizes of 20 to 25 nm. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH₃ and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42?ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3?MPa for fuel input power of 31.44?kW.     

ToF-SIMS study of chemical composition and formation of all-nanoparticle multilayer films

All-nanoparticle multilayer films were prepared by layer-by-layer deposition of SiO₂ and Al₂O₃ nanoparticles onto polyester (PE) substrate. The top-most SiO₂ (and Al₂O₃) layer was characterized using ToF-SIMS and SEM. An element-specific homogeneity index obtained by ToF-SIMS measurement provides clue to the formation mechanism. Experimental results from ToF-SIMS and SEM accord well with molecular dynamics simulation results, demonstrating the potential of using ToF-SIMS to study all-nanoparticle multilayer films.